Dairy Chemistry & Milk Composition

Average Composition of Bovine Milk

Composition varies with breed, season, lactation stage, feed and milking management. Typical pH is 6.6–6.8; freezing point −0.530 to −0.540°C.

Milk Proteins

Milk has approximately 3.3–3.5% total protein, split between caseins (~80%) and whey proteins (~20%):

Caseins

The casein family consists of four phosphoproteins, all heat-stable but pH-sensitive:

• α_s1-casein (~38% of total casein) — main protein in casein micelle structure
• α_s2-casein (~10%) — similar function
• β-casein (~36%) — key flavour precursor during cheese ripening
• κ-casein (~13%) — surface protein on casein micelle; cleaved by rennet to initiate cheese coagulation

Caseins form spherical aggregates called casein micelles (50–500 nm diameter) held together by calcium phosphate "nano-clusters". The micelles are stable to heat (survive pasteurisation and even UHT) but coagulate at their isoelectric point (pH 4.6) or when κ-casein is cleaved by chymosin/rennet.

Whey proteins

Whey proteins remain dissolved when casein precipitates. They are denatured by heat above ~75°C:

• β-lactoglobulin (~50% of whey protein) — not present in human milk; major factor in cooked flavour and milk allergies
• α-lactalbumin (~20%) — calcium-binding; less prone to heat denaturation
• Bovine serum albumin (BSA) (~10%) — large protein; minor functional role
• Immunoglobulins (IgG) (~10%) — very high in colostrum; reduce over lactation
• Lactoferrin, lactoperoxidase, lysozyme — minor antimicrobial proteins; valuable as isolated ingredients

Denatured whey proteins (heated above ~75°C) bind to casein micelles via disulphide bonds — the chemistry behind yogurt structure and the impact of high-heat pasteurisation on cheese yield.

Milk Fat

Milk fat (3.5–4.2% in UK cow milk) exists as milk fat globules — spherical particles 0.1–10 µm diameter (average 3–4 µm) surrounded by a complex membrane (the MFGM — milk fat globule membrane).

Triglyceride composition

Milk fat is approximately 98% triglycerides, with ~400 different fatty acids detected. Major fatty acids:

The short-chain fatty acids (C4–C10) are unique to ruminant milk and drive distinctive flavour. They are also the source of "rancid" off-flavour when released by lipolysis (lipase action). The mix of saturated and unsaturated fatty acids gives milk fat its characteristic wide melting range (−40 to 40°C) and complex polymorphic crystal behaviour.

Milk fat globule membrane (MFGM)

The MFGM is a tri-layer structure (~8–10 nm thick) of phospholipids, glycoproteins, cholesterol and enzymes. It stabilises the fat-in-water emulsion and influences:

• Flavour and aroma transport
• Digestion (rate and completeness)
• Susceptibility to oxidation and lipolysis
• Behaviour in homogenisation (disrupted) and creaming (intact required)

Lactose

Lactose is the main milk sugar (4.6–4.9% in cow milk; ~6.5% in human milk). It is a disaccharide of glucose and galactose linked by a β-1,4 bond.

Key lactose properties

• Solubility — relatively low (~22 g/100 mL water at 20°C). Crystallises out at high concentrations or low temperatures (e.g. in concentrated milk powders, sweetened condensed milk — "sandiness" defect)
• Reducing sugar — reacts with proteins via the Maillard reaction during heating, producing the "cooked" flavour and brown colour of UHT, condensed and dry milk
• Fermented by lactic acid bacteria — the basis of yogurt, cheese culture activity, sour cream
• Hydrolysed by lactase enzyme — produces glucose + galactose; basis of lactose-free milk production
• Lactose intolerance — insufficient lactase enzyme in some adult populations; affects ~65% of global adult population to varying degrees

Minerals (Salts)

Milk contains 0.7% minerals (called "ash" after combustion). Most are dissolved in the serum phase, but calcium phosphate is split between dissolved and bound forms (associated with casein micelles).

Calcium balance is particularly important — pasteurisation drives some calcium out of solution into colloidal calcium phosphate, which impairs rennet coagulation. Adding CaCl₂ back (10–30 g/100 L) restores cheesemaking performance.

Enzymes in Milk

Raw milk contains many enzymes; some are heat-stable, some are easily destroyed by pasteurisation:

Physical-Chemical Properties Affecting Processing

Composition Variation by Species

The wide variation in composition across species is the reason infant formula manufacturers have to fortify cow milk substantially — cow milk is much higher in protein (mainly casein) and much lower in lactose than human milk. See milk powder and infant formula.

Frequently Asked Questions

What is milk made of?

Bovine milk is approximately 87.5% water, 3.5–4.2% fat, 3.3–3.5% protein (80% casein + 20% whey protein), 4.6–4.9% lactose (milk sugar), and 0.7% minerals (mainly calcium, phosphorus, potassium, sodium). Plus trace vitamins, enzymes and other components.

What's the difference between casein and whey protein?

Caseins (~80% of milk protein) form spherical micelles held together by calcium phosphate. They coagulate at pH 4.6 (isoelectric point) or when cleaved by rennet — the foundation of cheese making. Whey proteins (~20%) remain dissolved during coagulation but are denatured by heat above ~75°C. The main whey proteins are β-lactoglobulin and α-lactalbumin.

Why is calcium important in dairy processing?

Calcium is critical for casein micelle structure and rennet coagulation. Pasteurisation drives some calcium out of solution into colloidal calcium phosphate, weakening cheesemaking performance. Adding CaCl₂ (10–30 g/100 L) back to cheese milk restores coagulation strength. Calcium balance also drives cheese ripening behaviour and feta dissolution in brine.

What causes the cooked flavour in heated milk?

The Maillard reaction between lactose (a reducing sugar) and milk proteins (mainly β-lactoglobulin) during heating. Produces a range of flavour compounds and a brown colour. More intense at higher temperatures (UHT, condensed milk, powders) and longer times. Sulphur compounds from whey protein denaturation also contribute.

Why does milk go rancid?

Rancidity in milk comes in two forms: lipolytic (caused by lipase enzymes breaking down fat into free fatty acids — the short-chain ones C4–C10 give the distinctive "rancid" smell) and oxidative (caused by oxygen reacting with unsaturated fats, producing aldehydes and ketones). Lipolytic is mainly a raw-milk handling issue; oxidative is a packaging/light issue.

What is the pH of milk?

Fresh raw milk has pH 6.6–6.8. Below 6.5 suggests bacterial fermentation has begun. Below 5.0 indicates significant lactic acid development. Pasteurised milk drops slightly to 6.5–6.7. Yogurt fermented to pH ~4.6 (casein isoelectric point). Cheese drains at various pH points depending on the variety.

Why does buffalo mozzarella taste different from cow mozzarella?

Buffalo milk has roughly twice the fat (7–9%) and 50% more protein (4.5–5%) than cow milk. The higher fat content gives richer flavour; the different fatty acid profile (more saturated, different short-chain) gives a more distinctive aroma. The higher protein gives a firmer, more elastic texture. The PDO restricts genuine Mozzarella di Bufala to buffalo milk from specific Campania regions.

Further reading: John Watson publishes articles on dairy industry topics on LinkedIn. Browse all articles by John Watson on LinkedIn →

Related calculator: Practical calculations using the Pearson's Square method are commonly used in dairy plant operations for fat blending.

See related: Milk pasteurisation, Homogenisation, Cheese making, Milk powder & infant formula, Cream production, UK milk production, Dairy laboratory testing, Milk grading, Stokes' Law, all dairy science information, consultancy services.